1. 02 Feb, 2015 1 commit
    • Viresh Kumar's avatar
      cpufreq: Set cpufreq_cpu_data to NULL before putting kobject · 6ffae8c0
      Viresh Kumar authored
      In __cpufreq_remove_dev_finish(), per-cpu 'cpufreq_cpu_data' needs
      to be cleared before calling kobject_put(&policy->kobj) and under
      cpufreq_driver_lock. Otherwise, if someone else calls cpufreq_cpu_get()
      in parallel with it, they can obtain a non-NULL policy from that after
      kobject_put(&policy->kobj) was executed.
      
      Consider this case:
      
      Thread A				Thread B
      cpufreq_cpu_get()
        acquire cpufreq_driver_lock
        read-per-cpu cpufreq_cpu_data
      					kobject_put(&policy->kobj);
        kobject_get(&policy->kobj);
      					...
      					per_cpu(&cpufreq_cpu_data, cpu) = NULL
      
      And this will result in a warning like this one:
      
       ------------[ cut here ]------------
       WARNING: CPU: 0 PID: 4 at include/linux/kref.h:47
       kobject_get+0x41/0x50()
       Modules linked in: acpi_cpufreq(+) nfsd auth_rpcgss nfs_acl
       lockd grace sunrpc xfs libcrc32c sd_mod ixgbe igb mdio ahci hwmon
       ...
       Call Trace:
        [<ffffffff81661b14>] dump_stack+0x46/0x58
        [<ffffffff81072b61>] warn_slowpath_common+0x81/0xa0
        [<ffffffff81072c7a>] warn_slowpath_null+0x1a/0x20
        [<ffffffff812e16d1>] kobject_get+0x41/0x50
        [<ffffffff815262a5>] cpufreq_cpu_get+0x75/0xc0
        [<ffffffff81527c3e>] cpufreq_update_policy+0x2e/0x1f0
        [<ffffffff810b8cb2>] ? up+0x32/0x50
        [<ffffffff81381aa9>] ? acpi_ns_get_node+0xcb/0xf2
        [<ffffffff81381efd>] ? acpi_evaluate_object+0x22c/0x252
        [<ffffffff813824f6>] ? acpi_get_handle+0x95/0xc0
        [<ffffffff81360967>] ? acpi_has_method+0x25/0x40
        [<ffffffff81391e08>] acpi_processor_ppc_has_changed+0x77/0x82
        [<ffffffff81089566>] ? move_linked_works+0x66/0x90
        [<ffffffff8138e8ed>] acpi_processor_notify+0x58/0xe7
        [<ffffffff8137410c>] acpi_ev_notify_dispatch+0x44/0x5c
        [<ffffffff8135f293>] acpi_os_execute_deferred+0x15/0x22
        [<ffffffff8108c910>] process_one_work+0x160/0x410
        [<ffffffff8108d05b>] worker_thread+0x11b/0x520
        [<ffffffff8108cf40>] ? rescuer_thread+0x380/0x380
        [<ffffffff81092421>] kthread+0xe1/0x100
        [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
        [<ffffffff81669ebc>] ret_from_fork+0x7c/0xb0
        [<ffffffff81092340>] ? kthread_create_on_node+0x1b0/0x1b0
       ---[ end trace 89e66eb9795efdf7 ]---
      
      The actual code flow is as follows:
      
       Thread A: Workqueue: kacpi_notify
      
       acpi_processor_notify()
         acpi_processor_ppc_has_changed()
               cpufreq_update_policy()
                 cpufreq_cpu_get()
                   kobject_get()
      
       Thread B: xenbus_thread()
      
       xenbus_thread()
         msg->u.watch.handle->callback()
           handle_vcpu_hotplug_event()
             vcpu_hotplug()
               cpu_down()
                 __cpu_notify(CPU_POST_DEAD..)
                   cpufreq_cpu_callback()
                     __cpufreq_remove_dev_finish()
                       cpufreq_policy_put_kobj()
                         kobject_put()
      
      cpufreq_cpu_get() gets the policy from per-cpu variable cpufreq_cpu_data
      under cpufreq_driver_lock, and once it gets a valid policy it expects it
      to not be freed until cpufreq_cpu_put() is called.
      
      But the race happens when another thread puts the kobject first and updates
      cpufreq_cpu_data before or later. And so the first thread gets a valid policy
      structure and before it does kobject_get() on it, the second one has already
      done kobject_put().
      
      Fix this by setting cpufreq_cpu_data to NULL before putting the kobject and that
      too under locks.
      Reported-by: default avatarEthan Zhao <ethan.zhao@oracle.com>
      Reported-by: default avatarSantosh Shilimkar <santosh.shilimkar@oracle.com>
      Signed-off-by: default avatarViresh Kumar <viresh.kumar@linaro.org>
      Cc: 3.12+ <stable@vger.kernel.org> # 3.12+
      Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
      6ffae8c0
  2. 30 Jan, 2015 5 commits
  3. 23 Jan, 2015 33 commits
  4. 18 Jan, 2015 1 commit